Pneumatic casing tools are gripping devices used to hold and lower tubular well casing into a pre-drilled hole. The tools are used in sets consisting of one elevator slip assembly and one spider slip assembly. The elevator and spider slip assemblies are identical tools except for the accessories used with each tool.
The first problem associated with the use of these tools is related to gripping the casing collar which is of a larger diameter than the nominal diameter of the well casing. The problem is caused by not lowering the elevator slip assembly far enough below the collar. Such slip assemblies are designed such that the gripping forces generated are sufficient for proper gripping only when the slips are lowered sufficiently below a casing collar as to completely grip the nominal diameter of the well casing. When the collar is gripped, the slips are not allowed to go sufficiently deep into the tool body to generate adequate gripping forces. The result is that, when the casing string is lifted with the slips gripping the collar rather than the casing diameter, the drill string will sometimes slip through the slips and drop into well bore below.
The next problem is caused by improper operation of the gripping tools. The person working up in the derrick, called the "stabber", operates the control valve that closes the elevator slips. Once the elevator slips are closed and the weight of the casing string is on the elevator, the stabber sometimes actuates the control valve to the open direction. However, with the string weight hanging on the elevator, the air pressure alone will not open the slips. The proper time to actuate the control valve is after the string is lowered and the spider assembly slips are closed, and not before.
There is an instance when this is a problem. This instance would occur when the pipe is being lowered into the well bore and meets up with some restriction or abutment which prevents downward movement of the casing string. The traveling block and elevator continue to move downward a short distance because of the reaction time of the driller. This situation is a problem when the slips have been actuated in the open direction but have been held down by the weight of the casing string. The weight is no longer on the elevator and the slips consequently open up. If the casing string should suddenly free itself in this manner and drop, neither the spider nor the elevator are in the closed position and the casing string accordingly just drops into the bore hole.
The pertinent and presently known prior art to this invention are U.S. Pat. Nos. 3,215,203; 3,708,020; and 3,722,603.